1. Field of the Invention
The present invention relates to the field of communications. More specifically, it relates to a system and method for improving wireless data transmission by combining an adaptive antenna system with mobile location technology.
2. Description of the Related Art
Wireless data communication with mobile users is typically provided via fixed base stations or cell sites. Wireless data that may be communicated include electronic mail transmissions to and from a mobile unit, downloading a website from a remote Internet server, etc. Each cell site includes one or more antennas arranged for transmission of signals to and from mobile user equipment in the vicinity of the cell site. The signals received from the mobile user equipment may have a relatively low power.
To provide the desired antenna coverage, the area around the cell site may be divided into sectors. Four sectors each of 90 degrees in azimuth coverage, or three 120 degree sectors, thus provide 360 degrees azimuth coverage around a cell site. In some applications, a single sector may be expanded to provide full 360 degree azimuth coverage. The cell configuration reflects the necessity of providing a pattern of adjacent cells, each with its own cell site antenna installation, in order to provide mobile communications coverage over a geographic area or region larger than the limited area covered by a single such installation.
It is relatively straightforward to design an antenna system that provides 120 degree sector coverage. The system may be used to transmit signals to any user within the desired sector coverage area. In general, the power of the received signal for fixed transmitted power is inversely proportional to the width of the beam. If, for example, signals are too weak to reliably reach certain users located at an edge of the coverage area, transmitter power can be increased to a higher level. However, power amplifiers to support higher transmission power are expensive. If the user's transmitted power is inadequate to achieve reliable reception from the outer portions of the sector coverage area, the overall size of the cell and the gain of the receive antenna at the fixed cell site become limiting considerations.
A number of interrelated factors, including expected user population and available channel bandwidths, may also be involved in determining the optimum size of the coverage area of each cell site. It will be apparent, however, that with smaller coverage areas additional cell site installations will be necessary to provide continuous coverage over a geographical region. Additional cell site installations may require the purchase, installation and maintenance of more equipment as well as increased requirements and possibly costs of site acquisition, interconnection facilities and system support. Beam forming techniques to improve performance are well known mitigating techniques.
U.S. Pat. No. 5,612,701 to Diekelman (“the '701 patent”) discloses an adaptive beam pointing method in which a satellite provides two types of beams in response to communication demands from mobile units. The first type of beam is an access beam which covers a wide area and is used for coupling a first group of mobile units with the satellite for initiating communication. The second type of beam is a service beam which couples a first group of mobile units, via a beam centered around what is referred to as “the centroid,” to the satellite and then couples the satellite to a second group of mobile units via a beam centered around its centroid in an area remote from the first mobile unit once access has been granted via the access beam. The satellite is said to be capable of detecting the location of the mobile units that are requesting a communication link.
The '701 patent does not address certain problems associated with terrestrial systems. Terrestrial communications systems differ significantly from satellite-based communications systems. For instance, the near/far effect with terrestrial systems is of much greater significance than with satellite-based systems. That is, relative to the satellite, located approximately 26,000 miles from earth, all mobile units within a given geographical area are perceived as being substantially equidistant from the satellite. The effects are very different for a terrestrial system in which the respective distance from the base for mobile units may vary greatly. Another phenomenon present with terrestrial systems that is not addressed by the '701 patent is the fact that path losses may be different for the forward link and the reverse link due to propagation environment in the two frequencies e.g., shadowing effects and fast fading. Finally, terrestrial systems are subject to multiple paths unlike satellite systems.
In addition, the '701 patent does not refer to data communications. Data transmission is very different from speech transmission (as addressed in the '701 patent) in several respects. First, the request can come from either the base unit (reverse link) or the mobile unit (forward link), whereas the '701 patent teaches only the reverse link. Second, the forward and reverse links of a data transmission are asymmetrical by their nature. That is, information (e.g., packetized data) transmitted on the forward link might be much more intensive than the information transmitted on the reverse link at any one time and maybe opposite at another time. Third, packet data is inherently bursting (i.e., discontinuous) and the data rate for a particular beam can change instantaneously. That is not the case for speech applications such as the one described by the '701 patent.
Thus, there is a need for a system that allows for the independent control of and the customization of reverse link beams and forward link beams that couple a particular mobile unit with a base station for transmitting/receiving data. Ideally, the forward and reverse link beams would be dynamically customizable for data rate requirements changing with time, the position of the mobile unit relative to the base changing with time, and the SNR changing with time and wherein the customized beam can be steered directly to the mobile unit with which the base station is communicating.